Optical Study of the Free-Carrier Response of ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ Superlattices

We used infrared spectroscopic ellipsometry to investigate the electronic properties of ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface $\approx 3\times {10}^{14}{\mathrm{cm}}^{-2}$. This is probably due to the leakage of $d$ electrons at interfaces from the Mott insulator ${\mathrm{LaTiO}}_3$ to the band insulator ${\mathrm{SrTiO}}_3$. We observed a carrier relaxation time $\approx 35\mathrm{fs}$ and mobility $\approx 35{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 10 K, and an unusual temperature dependence of carrier density that was attributed to the dielectric screening of quantum paraelectric ${\mathrm{SrTiO}}_3$.

Figure 1

(a) RHEED intensity oscillation during the initial growth of the (LTO)2/(STO)10 SL. (b) Atomic force microscope topographic image ($5\times 5\mu {\mathrm{m}}^2$) of an (LTO)1/(STO)10 SL 100 nm thick with single unit cell terraces conserved. (c) X-ray $\theta \mathrm{\text{−}}2\theta$ scan of (LTO)1/(STO)10. Peaks marked with (*) are reflected from the STO substrate.

Figure 2

(a) Optical conductivity spectra of $(\mathrm{LTO})\alpha /(\mathrm{STO})10$ at 10 K (with vertical offsets), containing the coherent (Drude) and incoherent (Lorentz) contributions. (b) SL real dielectric constant spectra as a function of photon wave number at 10 K. The arrow indicates the screened plasma frequency ${\omega }_p^{*}[={\omega }_{pD}/\sqrt{{\epsilon }_{\infty }}]$. (c) $T$-dependent ${\sigma }_1(\omega )$ of (LTO)2/(STO)10.

Figure 3

(a) $n_{\mathrm{sheet}}$ per IF. The solid line indicates the value of an ideal 0.5 $e^{-}$ per unit cell area, and the dashed and dotted lines represent theoretically predicted values reported by Kancharla [5] and Okamoto [3], respectively. The vertical arrow shows the range of $n_{\mathrm{sheet}}$ in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_{3-\delta }$ IF from Ref. 23. (b) Comparison of SL ${\omega }_{pD}$ with solid solution [17] and the ideal doping of $[\mathrm{La}]=n$ as a function of La and Sr composition.

Figure 4

$T$ dependence of (a) the carrier density, and (b) the relaxation time and mobility of free carriers for $(\mathrm{LTO})\alpha /(\mathrm{STO})10$ SLs.